In automobile engines, a lubricating oil is heated and exposed to a blowby gas during a long period of operation, so that the lubricating oil is contaminated with unburned hydrocarbons and degenerated oil additives (called “oil sludge” as a whole). The oil sludge includes oil sludge precursors having relatively high viscosity. The oil sludge attached to and accumulated on engine parts likely wears the parts and clogs lubricating oil paths, causing troubles in the functions of engine parts such as combined oil control rings (called “oil rings” unless otherwise mentioned). In the oil rings, spacer expanders and side rails are stuck to each other in the worst case, hindering the movement of separate side rails, and thus failing to exhibit a sufficient oil-controlling function.
As shown in FIGS. 6(a)-6(c), a conventional combined oil ring 101 comprises a pair of annular side rails 120a, 120b arranged with a gap, and a spacer expander 101 supporting the side rails 120a, 120b. The spacer expander 101 in an axially corrugated form has upper portions 102, lower portions 103, and leg portions 104 connecting the upper portions 102 and the lower portions 103. It has tabs 105a, 105b on the inner side of the upper portions 102 and the lower portions 103, projections 106a, 106b supporting the side rails 120a, 120b on the outer side, and dented intermediate portions 107a, 107b between the tabs 105a, 105b and the projections 106a, 106b. When the spacer expander 101 is combined with the side rails 120a, 120b, each space 108a, 108b is defined by each tab 105a, 105b, each projection 106a, 106b, each intermediate portion 107a, 107b, and each side rail 120a, 120b. 
In the oil ring, the side rails 120a, 102b are pressed by radial and axial components of a force due to the inclination angle of the tabs 105a, 105b of the spacer expander 101, thereby exhibiting a function of sealing a cylinder wall and side surfaces of ring grooves of a piston. Particularly, a small-width oil ring having a small axial width h1 has good followability to a cylinder sidewall with a side-sealing function, resulting in reduced friction loss without increasing oil consumption even if it is a low-tension type. However, in the oil ring, oil sludge is easily accumulated in a space 108a, 108b between the spacer expander and the side rails as described above. Particularly in the case of a small-width oil ring with extremely narrow space 108a, 108b, the side rails 120a, 120b are highly likely stuck to the spacer expander 101 by the accumulated oil sludge. The accumulation of oil sludge extremely reduces the followability of side rails 120a, 120b to a cylinder wall, resulting in drastically increased oil consumption.
To prevent the attachment and accumulation of oil sludge to an oil ring, the surface coating of a spacer expander and a side rail, and the structure of a spacer expander avoiding the accumulation of oil sludge are disclosed.
For example, JP 2002-310299 A and JP 2003-254155 A disclose a fluororesin coating and a fluororesin-containing resin coating, JP 2000-027995 A discloses a coating containing fluoroalkyl-substituted alkoxide, JP 2006-258110 A discloses a hydrophilic prepolymer coating containing inorganic polysilazane, WO 2011/043364 A1 discloses a method for forming a metal coating having low surface free energy and hydrogen bonding force. These coatings are water-repellant, oil-repellant, or oppositely hydrophilic coatings, or those formed based on methods of preventing the attachment of oil sludge.
As a structural measure, JUM 59-127856 A discloses an oil ring comprising a spacer expander having upper and lower portions, sufficiently large holes permitting foreign materials such as lead compounds to pass being formed in intermediate portions of the upper and lower portions, such that they do not reach uprising portions of corrugation. U.S. Pat. No. 5,195,758 and JP 2011-185383 A disclose spacer expanders having structures preventing the accumulation of oil sludge, which comprise radial grooves in intermediate portions, and oil-exiting holes communicating with the grooves in tabs.
However, the coating methods of JP 2002-310299 A, JP 2003-254155 A, JP 2000-027995 A, JP 2006-258110 A, and WO 2011/043364 A1 suffer excess steps leading to cost increase, and the hole-forming method of JUM 59-127856 A need difficult working, making the oil rings expensive. Also, oil resides in the grooves formed in intermediate portions in U.S. Pat. No. 5,195,758 and JP 2011-185383 A during the stop of engines, likely suffering the accumulation of oil sludge. As a result, sufficient durability is not obtained in such an operation pattern as undergoing repeated stop of engines.